Various physical, chemical or physico-chemical methods are known for preparing dispersed metal phases. For example, the metal can be ground mechanically and then dispersed in the selected medium, or a metal which has been previously vaporized under a vacuum can be condensed onto a support. Also, such can be carried out by ion reduction with molecular hydrogen at a high temperature, or by attack with a base on a metal alloy, for example, for the preparation of Raney nickel. The physico-chemical methods can consist of reducing metal ions at the surface of an electrode, carrying out a discharge in a metal ion solution or carrying out a photochemical reduction.
However, these known methods do not yield the metal in the state of microaggregates uniformly dispersed in either a liquid or solid medium, and having a size of less than a few nanometers.
For example, German Pat. No. 1,154,442 describes the photochemical decomposition of an organic metal compound in a solution of an ethylene polymer in a hydrocarbon or an either in order to obtain a colloidal dispersion of lead, zinc, nickel or iron. As described in U.S. Pat. No. 4,264,421, a photochemical method also gives a metal deposit on a support. U.S. Pat. No. 1,805,199 describes the thermal decomposition of a metal derivative yielding colloidal lead or nickel particles which, in solution, exhibit the Tyndall effect characteristic of a particle size greater than 100 nm. German Pat. No. 1,717,152 describes the preparation of a nickel-based catalyst by high temperature vaporization of the metal in order to obtain particles of a size of between 30 and 45 nm.
The use of ionizing radiation has also been suggested (.gamma. or .chi. photons or accelerated electrons) in order to carry out in situ reduction of noble metal salts using solvated electrons formed in the solvent. This method has the advantage of producing solvated electrons at all points of the liquid, even inside the micropores of an alveolar support which is appropriately used. The metals thus prepared belong to the group of noble metals (Ir, Pt, Pd, Rh, Ru, Au, Ag) but only Ir, Pt, Au and Ag have been obtained in a divided form. Further, the method does not enable other metals, such as nickel, which are also often used in various catalysts for numerous reactions, to be obtained in a stable and very divided form. For example, U.S. Pat. No. 3,826,726 describes the reduction of metal ions, but a precipitate of particles having a large size is obtained.
These difficulties result from the fact that the non-noble metals are characterized by a redox potential which is more favorable to corrosion and which is further shifted towards unfavorable values for quasi-atomic aggregates. The production of metals in a state of extreme division can only succeed by avoiding, at least partially, the reverse reaction of corrosion during the nucleation phase of the aggregate. Beyond a certain size, the particles are stabilized with respect to the surrounding medium.
Another difficulty appears in the case of multivalent ions for which progressive radiolytic reduction is involved. Although this reduction method has the advantage of ensuring the dispersion of the native metal atoms and of enabling control of their aggregation, it has, like any method based on the reduction of ions, the risk of oxidation on the return of the intermediate state or states, which risk is in addition to the corrosion of the aggregates at the very first stage of their growth.